A well known problem in the art of digital optical communication was an inability to use burst-mode transmission, i.e., a direct unencoded transmission of data in which the data desired to be transmitted is allowed to have long strings of only ones or long strings of only zeros. This was because, for proper operation of the receiver, the non-transition period, i.e., the length of time between changes in the optical signal, had to have been be small relative to the time constant determined by the capacitance for coupling between a) a preamplifier of an electrically-converted version of the optical signal and b) a comparator which is used to determine the logic value, i.e., 1 or 0, of the incoming optical signal. As a result, an optical signal with a guaranteed maximum transition spacing had to be essentially constantly transmitted, which required that the data to be transmitted be encoded. Disadvantageously, doing so mandated that a decoder be present in the receiving system to remove the formatting and reconstruct the original data. Also, data transmission efficiency was degraded because of the required extra bits for the encoding.
Previously, it was recognized that the foregoing disadvantages resulted from the design of prior art optical receivers, which employed alternating current (AC) coupling between the optical preamplifier and the comparator. While such AC coupling is not always needed in electrical communication systems, it was believed to be required in optical systems, because there is no common electrical connection between the transmitter and the receiver. Nevertheless, eventually the above-noted disadvantages were overcome by burst-mode digital optical receivers which use direct current (DC) coupling. Such DC coupled burst-mode digital optical receivers are disclosed in U.S. Pat. Nos. 5,025,456, 5,430,766, and 5,371,763 issued to myself and R. G. Swartz, which are incorporated herein by reference. Such DC coupled optical receivers used a different approach than other prior art receivers, essentially unifying the preamplifier and the comparator, and making use of an automatic threshold controller (ATC). However, such an approach has the disadvantage of doubling the preamplifier noise, i.e., there is an inherent 3 dB noise penalty.